Phase structure and electrochemical properties of La0.75Mg0.25Ni3.5-xAlx (x=0∼0.3) hydrogen storage alloys of A2B7 type

Quaternary Hydrogen Storage Alloys La0.75Mg0.25Ni3.5-xAlx (x=0, 0.02, 0.06, 0.1, and 0.3) of A(2)B(7) type were prepared by induced melting method. The effects of partly replacing Ni by Al element in La0.75Mg0.25Ni3.5 on the phase structure and electrochemical properties of La0.75Mg0.25Ni3.5-xAlx quaternary alloys were investigated. XRD analysis shows that the La0.75Mg0.25Ni3.5 consists of single (La, Mg)(2)Ni-7 phase, the LaNi5 phases of CaCu5 type appears when Al was added to substitute Ni, and becomes the main phase when x=0.3. The Rietveld analysis shows that the increase of Al content leads to an increase of LaNi5 pliase. We can draw a conclusion that the Al element is helpful to the formation of LaNi5 phase. The electrochemical analysis shows that the substitutions of Al lead to some decrease of maximum discharge capacity and a little effect on the activation property. For a discharge current density of 1600 mA/g, the high rate discharge (HRD) ability of the alloy electrodes increases from 68.80% (x=0) to 81.16% (x=0.1), then decreases to 65.67% (x=0.3). The cyclic stability of alloy electrodes increases firstly, then decreases with increasing the Al content in alloys. The parameter S-100 for La75Mg0.25Ni3.5-xAlx (x=0, 0.02, 0.06, 0.1, and 0.3) are 73.77%, 77.69%, 85.21% 7 0.26% and 70.96%, respectively.